PROJECT SUMMARY/ABSTRACT Genomic instability is a hallmark of cancer and correlated with overall poor patient prognosis. A specific form of genomic instability is chromosomal instability (CIN), which occurs when a cell continuously missegregate their chromosomes leading to changes in karyotype. CIN is a common feature of cancer ? with approximately 90% of solid tumors and 50% of hematopoietic cancers exhibiting CIN. CIN is also correlated with increased tumor progression and poor patient prognosis. However, the levels of CIN must be maintained at an optimal level, because too high levels of missegregation events are deleterious to cells. This creates a therapeutic window by which chemotherapies can exploit CIN to create effective treatments against cancer cells. ATR, a master regulator of DNA damage repair pathways, ensures genomic stability by promoting DNA damage repair and timely replication. I recently showed that ATR also promotes genomic stability by promoting faithful chromosome segregation in mitosis and preventing CIN. In this application, I propose to investigate the mechanism by which ATR is activated in mitosis (Aim 1), identify its substrate network (Aim 2) and use ATR inhibition to target CIN cancer cells specifically (Aim 3). My proposed studies may reveal more novel functions of ATR in mitosis and allow us to better understand of how mitotic processes are regulated by ATR and other DDR proteins to ensure proper chromosome segregation. This in turn, will give us a better understanding of the overlap of mitotic and DDR proteins in ensuring genomic stability. Lastly, I hope that developing a strategy to specifically kill CIN cancer cells will be a breakthrough in targeted cancer therapies. My career goal is to obtain a research faculty position at a leading institution where I will dissect the mechanisms by which ATR and other DDR proteins regulate chromosome segregation. My successful transition would be bolstered by augmenting my expertise in cell biology, mass spectrometry and mouse cancer models. I will use these acquired skills to investigate whether ATR inhibitors can be used to specifically target CIN cancer cells. Importantly, the protected time that this award provides me will allow me to elucidate the mechanism by which ATR is activated and determine if ATR inhibitors can be used to exploit CIN in cancer cells. Furthermore, the success of this project will be greatly enhanced by the outstanding collaborators that I will have advise me through the K22 period. The receipt of this award will allow me to expand my research plan and establish myself as a primary investigator in the field of cancer biology.